Formation mechanism and control of surface peeling defect on a medium-carbon Cr–V system hot-rolled sheet

This work investigated the edge peeling defects observed in the hot-rolled sheets of a medium-carbon alloy spring steel (50CrV4). Combined experimental characterisation and numerical simulation were employed to elucidate the formation mechanism and control strategy of the defects. Optical microscopy and scanning electron microscopy revealed severe oxidation and abnormal decarburisation at the defect sites. The defects originated from the inheritance of transverse corner cracks in continuous casting slabs. Phase transformation calculations and high-temperature tensile tests further showed that 50CrV4 steel had poor ductility below 800 °C. This ductility loss, related to grain-boundary ferrite films and possibly fine V-containing carbonitrides, was considered a key factor in the formation of slab corner cracks. Numerical simulations of the heat transfer and solidification during continuous casting revealed that the slab corner temperature upon entering the straightening section of the continuous caster was approximately 775 °C under the original process, falling within its low-ductility trough and thus leading to the occurrence of cracks in the slab. Consequently, the secondary cooling strategy was proposed based on the heat-transfer simulations. By reducing the water flow rate in cooling zones 4 to 8 segments to 80% of the original one to optimise production, the corner temperature at the entrance of the straightening section was predicted to increase above 800 °C without excessively extending the metallurgical length. Industrial trials verified the effectiveness of the optimised process, where the defect rate of hot-rolled sheets dropped from 8.31% to 0.12%, significantly improving product quality. This study provides a theoretical basis and practical guidelines for surface quality control on hot-rolled sheets.